The present invention relates to a selective catalytic reduction system for reducing nitric oxide emission levels from a compression ignition engine, comprising a first reductant injector located upstream a first catalyst comprising a selective catalytic reduction coating.
In the art of combustion engines, there has been a growing concern regarding emissions, at least since the early 1970's. For SI (spark ignition) engines, the emissions can hardly be regarded as a problem after the three-way catalyst was put on the market in the mid 70:s. For CI (compression ignition) engines, the situation is, however, slightly more complicated; CI engines have inherently high emission levels of nitric oxides (NOx) and particles (soot). Due to the nature of the engine combustion in a CI engine, a large amount of air is namely inducted into the cylinders, whereupon the air is compressed. Thereafter, an amount of fuel varying as a function of engine load is injected into the compressed air. The injected fuel will auto-ignite due to the high temperature resulting from the compression of the air. The injected fuel will burn in a diffusion mode, i.e. the combustion speed will be more or less controlled by the mixing rate between the injected fuel and the compressed air. Soot will form in fuel rich zones and NOx will form in combustion zones where the temperature is high and enough oxygen is left to form NOx.
One obstacle concerning exhaust aftertreatment for reducing NOx in CI engine exhausts is the presence of oxygen; as implied earlier, a large amount of air is inducted in the cylinders prior to fuel injection. Hence, there is always a surplus of air in the cylinders, compared to the amount of air necessary to completely combust all the injected fuel. An air surplus in the exhausts makes it impossible to use a standard three-way catalyst in order to reduce NOx emissions. Soot (or particles) is/are also a major problem for CI engines; soot forms, as mentioned, in fuel rich combustion zones. High fuel injection pressures, that form small fuel droplets, can reduce soot formation significantly, but there are design limitations on how high injection pressure that can be accepted.
There are methods to reduce NOx formation in CI engines; the most common way is to delay injection timing. By delaying injection timing, the maximum combustion temperature can be reduced, which in turn will decrease NOx formation. The NOx reduction comes, however, with some severe penalties, namely that both soot formation and fuel consumption increase with later injection timing.
One efficient way of reducing NOx emissions is to use an SCR (Selective Catalytic Reduction) emission aftertreatment system. A common SCR system comprises a substrate coated with e.g. zeolites, V-oxides (e.g. V2Os), Cu-zeolites, Fe-zeolites or any other known material suitable for SCR. Unlike three-way catalysts for SI engines, SCR systems cannot work in an environment consisting of exhausts only; some additional agent, e.g. a reductant, must be added to the exhaust gas. A common such agent is urea, i.e. (NH2)2CO, as well as hydrocarbons or hydrogen.
The efficiency of an SCR catalyst, related to the driving cycle, is limited to about 65-80 percent, and its function is severely impaired by the presence of soot. For exhaust after-treatment of soot emission, a filter is used; such a filter does however require regeneration at intervals varying with engine load conditions. The regeneration mostly means that the exhaust temperature is increased by any means, e.g. by extremely late fuel injection, a post injection, by inlet air throttling, or by any other suitable means. The increased exhaust temperature allows soot particles trapped in the filter to “burn off”, i.e. react with oxygen in the exhausts to form carbon dioxide and water. However, every regeneration sequence results in a fuel economy penalty. Present and future legislation concerning N0χ emissions will make it virtually necessary to combine late fuel injection, SCR systems and soot filters. This leads to a “vicious circle”, ultimately resulting in a high fuel consumption, which in turn leads to an increased greenhouse effect and a bad driving economy.
If the NOx exhaust aftertreatment could be improved, then the vicious circle should be broken; the fuel injection timing could be set to a setting giving a minimum fuel economy, from which would follow decreased soot formation, which in turn would make NOx reduction easier.
It is desirable to provide an exhaust aftertreatment system enabling a high NOx conversion factor.
According to an aspect of the present invention a second reductant injector is located downstream the first catalyst and a second catalyst is placed downstream the second reductant injector and comprising a selective catalytic reduction coating.
In one aspect of the invention, the first catalyst comprises a filter function to trap particles formed by the CI combustion. In this aspect, the first catalyst could comprise a multitude of elongate cells with alternately closed and open top and bottom ends, respectively, wherein an exhaust gas flow is forced to pass through cell walls constituting the cells, and wherein the selective catalytic reduction coating is coated on either or both sides of the walls.
In order to further provide the system according to the invention with an oxidizing capability, an oxidizing catalytic coating could be coated on an upstream side of the cell walls.
For fine-tuning of the amount of reductant to be injected by the reductant injectors, sensors sensing presence of nitric oxides and/or ammonia in the exhaust gas could be arranged in the exhaust gas stream. One embodiment of the fine-tuning comprises placing the sensors downstream the filter function and downstream the second catalyst, respectively. Another embodiment comprises just one sensor for NOx or NH3 being placed downstream the second SCR catalyst.